With multiple ResourceHandlerRegistrationCustomizer beans in the context, only one of them is used

[kzander91]

WebFluxAutoConfiguration injects ResourceHandlerRegistrationCustomizer using ObjectProvider.getIfAvailable():

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfiguration.java

Line 181 in 8964203

this.resourceHandlerRegistrationCustomizer = resourceHandlerRegistrationCustomizer.getIfAvailable();

When multiple ResourceHandlerRegistrationCustomizer are in the context, this causes the WebFlux auto config to use an arbitrary one of them, instead of all of them.

In my case, webjars-locator-light joined my classpath due to a version update of another library. Due to this, @ConditionalOnEnabledResourceChain started matching, causing Boot to define its own customizer bean here:

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/WebFluxAutoConfiguration.java

Lines 351 to 361 in 8964203

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnEnabledResourceChain
	static class ResourceChainCustomizerConfiguration {
	@Bean
	ResourceChainResourceHandlerRegistrationCustomizer resourceHandlerRegistrationCustomizer(
	WebProperties webProperties) {
	return new ResourceChainResourceHandlerRegistrationCustomizer(webProperties.getResources());
	}
	}

This instance is the one that ends up being used, and my own is ignored.

I propose to allow an arbitrary number of customizer beans to work here, similar to the various other customizer callback interfaces used in Spring Boot's auto-configuration.

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I'm currently working around this issue by marking my bean as @Primary.

[wilkinsona]

Closing in favor of #43494. Thanks for the PR, @nosan.